This invention relates to novel planar materials having band gap properties, and in particular to such materials formed with fractal patterns.
Band gap materials are materials that have a gap in the transmission band through which electromagnetic radiation will not be transmitted. Such materials are conventionally constructed as three-dimensional crystal structures known as photonic crystals designed to give a desired photonic band gap. Such photonic band gap materials have a large number of potential applications. However, conventional photonic band gap materials must be fabricated as a composite material with a modulation of the dielectric properties. Because the band gap is caused by Bragg scattering within the crystal, this modulation must be of the same order of the wavelength of the band gap. For example, for optical photonic crystals there must be microstructures of the order of 0.1 microns, which makes them extremely difficult and costly to fabricate. On the other hand photonic crystals designed to work in the radio or microwave spectrum would have sizes in the range of a few centimeters or more, which would often make them too large and bulky for practical applications. For example, a photonic crystal with a band gap centered around 0.9 GHZ would make a perfect shield for mobile phones (for example for isolating a user from any potentially harmful radiation), except that the photonic crystal would have to be larger than the phone itself. For reasons such as these, photonic materials have yet to be used on a widespread basis.
Fractal patterns have been known for a number of years in mathematics. They have proved to be a useful tool in the analysis of mathematically complex and chaotic situations. They have yet, however, to find widespread practical applications in the physical sciences. A number of recent patents, however, attempt to find applications for fractal patterns in the field. For example, U.S. Pat. No. 6,127,977 (Cohen) describes a microstrip patch antenna formed with a fractal structure on at least one surface of a substrate. U.S. Pat. No. 6,140,975 (Cohen) describes an antenna structure with a fractal ground counterpoise and a fractal antenna structure. U.S. Pat. No. 6,104,349 discusses tuning fractal antennas and fractal resonators.
According to the present invention there is provided a planar bandgap material comprising a conductive fractal pattern formed on a non-conducting planar substrate.
The fractal pattern may be formed with any number of levels, but between 2 and 15 levels may be sufficient. The low-frequency limit of the bandgap(s) possessed by the material is determined by the number of levels of said fractal pattern, as well as the size and the geometry of the fractal pattern in each level
In preferred embodiments the fractal pattern is formed by subjecting a mother element to a repeated affine transformation. This mother element may be an H-shape and said transformation comprises scaling and rotation. However, it should be noted that the mother element does not have to be an H-shape and other possible shapes may be employed. Preferably, however, the mother element is a shape such that when it is subject to an affine transformation by scaling and rotating repeatedly to form the fractal pattern, the resultant pattern is xe2x80x9cself-avoidingxe2x80x9d so that the conductive elements do not run into each other or overlap. Other possible shapes for the mother element include a Y-shape, a V-shape and the shape of a tuning fork.
Preferably the fractal pattern is embedded within a dielectric material.
More prferably still there may be provided means for injecting a current into the fractal pattern so as to alter the bandgap properties of said material.
Viewed from another aspect the present invention provides a planar bandgap material comprising a conductive fractal pattern formed on a non-conducting planar substrate and having at least one bandgap wherein all the dimensions of the material are smaller than the wavelength at said bandgap.
Viewed from a still further aspect the invention provides an electromagnetic radiation shield comprising a conductive fractal pattern formed on a substrate.
The present invention also extends to a method of forming a bandgap material comprising depositing a conductive fractal pattern on a planar substrate, and wherein the locations of the bandgaps are controlled by selecting the dimensions of a mother element of said pattern and the number of levels of said pattern.
The method may further comprise embedding said fractal pattern in a dielectric substrate.
The method of forming a bandgap material may further comprise providing means for injecting a current into said pattern whereby the bandgap properties of said material may be altered.
Viewed from a further aspect the present invention provides a narrow-band electromagnetic filter comprising a wire mesh material adjacent to a plate formed with a conducting fractal pattern thereon.